1. Field of the Invention
The present invention relates to a birefringent, reflecting optical element provided with a sub-wavelength metal structure and an optical apparatus using the same.
2. Background Art
Optical apparatuses are widely used and optical elements for controlling light are often used for optical information communication apparatuses, displays, optical pickups, optical sensors or the like. As the functions of these apparatuses become more sophisticated, the optical elements are also required to have higher functions, higher added values and lower costs.
As an example of such optical apparatuses, “JP 2008-224313 A (corresponding to U.S. 2008/0218836 A)” discloses a demodulator technique of reducing deterioration of signal quality caused by PDFS (Polarization Dependent Frequency Shift) which occurs at the time of demodulation under a phase modulation scheme as a receiving apparatus of optical information communication with a phase compensation element. “JP 2008-122618 A” discloses a technique of improving optical utilization efficiency of a light source for a projector (display) using a metal structure smaller than a wavelength. “JP 2008-65961 A (corresponding to U.S. 2008/0067321 A)” discloses an optical pickup technique of improving an S/N ratio by causing reference light and signal light to interfere with each other using a homodyne scheme. These optical apparatuses realize desired functions by combining a plurality of reflecting mirrors, beam splitters for switching between optical paths according to the state of polarization of light and wave plates for converting the state of polarization of light or the like.
As a beam splitter taking advantage of a difference in the state of polarization of light, a polarized beam splitter using an optical multi-layer film, wire grid having a comb-like grid structure of metal wire arranged at an interval smaller than the wavelength or the like are known. Examples of the wave plate include one using optical anisotropic crystal represented by crystal and calcite and crystal and one having a dielectric comb-like grid structure arranged at an interval smaller than the wavelength as disclosed in “Applied Optics, 41, 3558 (2002)” and “WO 2007-055245 A1 (corresponding to U.S. 2009/0128908 A).” “Science, 305, 788 (2004)” comments on a technique about a meta-material whose refractive index is artificially controlled and negative refraction using a structure mainly made of metal which is smaller than the wavelength. Furthermore, “JP 2001-215462 A” discloses a technique on a high polymer film provided with a wave plate function mainly for a display. Furthermore, “JP 2004-170623 A” discloses a technique on a phase difference plate using a high polymer film.
“JP 2008-224313 A (corresponding to U.S. 2008/0218836 A)” to “JP 2008-65961 A (corresponding to U.S. 2008/0067321 A)” realize the functions as the respective optical apparatuses by combining a plurality of reflecting mirrors, optical elements having a polarized beam splitter function which switches between optical paths according to the state of polarization, optical elements having a wave plate function or the like. Here, it goes without saying that if a small and inexpensive new optical element that integrates a reflecting mirror and a wave plate function is realized, it is possible to reduce the size and cost of these optical apparatuses.
The wave plate is most expensive among the aforementioned optical elements. Conventional wave plates use birefringent optical anisotropic crystal, processed to a predetermined thickness. The optical anisotropic crystal has different refractive indexes between specific polarization (ordinary light) and polarization perpendicular thereto (non-ordinary light), and calcite as a typical example has a refractive index difference Δn of 0.17 at a wavelength 633 nm. By contrast, “Applied Optics, 41, 3558 (2002)” and “WO 2007-055245 A1” realize a wave plate (described as “polarization separation element” in “WO 2007-055245 A1”) by applying micromachining to a dielectric material such as glass using a semiconductor process without using expensive optical anisotropic crystal. The pitch of such a dielectric microstructure needs to be smaller than the wavelength in order to avoid branching of incident light by diffraction. Furthermore, since the refractive index difference Δn is on the order of 0.2, the required aspect ratio of the comb-like structure is said to be equal to or above 7. If the aspect ratio of the comb-like structure is equal to or below 1, it is possible to manufacture the wave plate at low cost through an injection molding process used for CD and DVD or the like without using any semiconductor process requiring a large-scale manufacturing apparatus, but this cannot be realized by only the techniques disclosed in “Applied Optics, 41, 3558 (2002)” and “WO 2007-055245 A1.” In comparison with these, the refractive index difference Δn between orthogonal polarizations of a liquid crystal material, which is said to be large, is on the order of 0.2 to 0.3.
On the other hand, the wave plates using a high polymer material described in “JP 2001-215462 A” and “JP 2004-170623 A” can provide low-cost members of large areas and are therefore mainly suitable for a display. However, these wave plates use a high polymer material, and cannot thereby surpass elements using an inorganic material in terms of performance and environmental resistance and are hardly applicable to the optical information communication apparatus and optical pickup described in “JP 2008-224313 A (corresponding to U.S. 2008/0218836 A)” and “JP 2008-65961 A (corresponding to U.S. 2008/0067321 A).”
In view of the above described problems of the conventional optical elements, it is an object of the present invention to provide a small and inexpensive new optical element which integrates a reflecting mirror, wave plate function or the like.